A sandboxed message is a message sent through a Sandbox server.
These messages are sent to a black hole, meaning they're never sent to the recipient. That means your test messages are visible and will appear as Delivered in Postmark’s UI, webhooks, API endpoints, etc., but won’t actually reach a real inbox.
Even though test messages aren’t delivered to real inboxes, we still process them through Postmark’s infrastructure and our API remains accessible for these messages. That’s why test emails count towards your monthly sending volume.
Postmark's developer user guide covers Sandbox mode in further detail.